1. Field of the Invention
This invention relates to a process for producing polyethylene and an ethylene-.alpha.-olefin copolymer, particularly to deactivation of a coordination polymerization catalyst to be used for polymerization of ethylene and .alpha.-olefins.
2. Description of the Prior Arts
Polyethylene and an ethylene-.alpha.-olefin copolymer polymerized by use of a coordination polymerization catalyst have generally a broad range of densities from 0.850 to 0.975 g/cm.sup.3, and utilized in large amounts for a wide variety of uses, including, for example films, blow moldings, fibers, extruded articles, etc.
As the catalyst for polymerization of ethylene or a mixture of ethylene with .alpha.-olefin, coordination polymerization catalysts have been known. Coordination polymerization catalysts include transition metal compounds belonging to the groups IV to VIII of the periodic table, typically of titanium, vanadium, chromium and molybdenum, and organometallic compounds such as organic aluminum compounds as principal constituting components.
Various processes have been known for polymerization of ethylene and a mixture of ethylene with an .alpha.-olefin. Among them, the solution polymerization process, in which polymerization is conducted at a high polymerization temperature of 130.degree. C. or higher or the high temperature high pressure ionic polymerization process using no solvent, which enables adiabatic polymerization of ethylene, is an excellent energy-saving process, since no energy is required for removal of polymerization heat, as different from the slurry polymerization process and the gas-phase polymerization process.
In recent years, high activity coordination polymerization catalysts have been developed, and the polymers obtained by use thereof are very small in quantity of the catalyst residue in the polymer even without removal of the catalyst residue in the polymer by extraction or neutralization with an alcohol or a caustic soda, and comparable in color or heat stability to the polymers of the prior art applied with removal of catalysts. When there is a process for removing a catalyst, the polymerization solvent and the unaltered monomers recovered cannot be used as such for polymerization, because they are in contact with polar compounds such as alcohols, but these polar compounds are required to be separated in the purification step. On the other hand, when a highly active catalyst is used, no polar compound such as an alcohol is employed, and therefore re-use of the polymerization solvent and the unaltered monomers may be possible without purifying a part or all of the polymerization solvent and unaltered monomers at all or only with treatment according a very simple purification step (e.g. passing through a molecular sieve), whereby enormous amounts of energy such as steam required for distillation purification can be saved.
However, in a process of high temperature polymerization with a coordination polymerization catalyst, polymerization rate is very high. Hence, if the step for removal of the catalyst is omitted, the catalyst is not deactivated and polymerization after emerging from the reactor, namely post-polymerization will occur. Post-polymerization, which will generally occur at a polymerization temperature higher than the average temperature in the polymerization reactor, is a cause for formation of undesirable low molecular weight oligomers, waxes and greases. Oligomers such as butene-1, hexene-1 and the like may cause lowering in density during preparation of ethylene homopolymers.
On the other hand, in a high temperature high pressure ionic polymerization process, polymerization conversion of ethylene is as low as 10 to 30%, and a large amount of unaltered monomers exist in the polymer emerging from the polymerization reactor, which will undergo polymerization with no reaction control when the catalyst is not deactivated, whereby there is involved a great risk of causing a runaway reaction.
Japanese Laid-open Patent Publication No. 111282/1976 discloses a proposal to use an aliphatic acid metal salt as the deactivator for the high temperature, high pressure ionic polymerization process, and also the same aliphatic acid metal salt is proposed to be used as the deactivator for the solution polymerization process in Japanese Laid-open Patent Publication No. 121004/1982.
However, when an aliphatic acid metal salt is used as the deactivator and the polymerization solvent and unaltered monomers recovered from polymer solution are used repeatedly without purification for a long term, the polymerization conversion is gradually lowered until polymerization occurs no longer. This is probably because the deactivator or decomposed products thereof, or the reaction products between the deactivator and the catalyst cannot sufficiently be separated in a separator downstream of the polymerization reactor, but entrained into the solvent and the unaltered monomers recovered to deactivate the activity of the catalyst.